Heat exchangers, particularly of the type used in automotive vehicles for engine cooling or heater purposes comprise a header and a tank at each end of the heat exchanger. It is vital that the junction of the tank and each header be leak-free, even over years of severe service and in the presence of somewhat corrosive fluids. It has become a practice to make the tanks of plastic or metal and join them to the metal headers. Thus it is important to provide sealing arrangements to joint tanks and headers subject to corrosive environments and temperature cycling in a manner to maintain seal integrity.
It is an accepted practice to use elastomer gaskets in the joints of the tanks and headers. Various rubbers such as EPDM or nitrile rubber are used for this purpose. The rubber is solid rather than foam and is used to fill the space between the tank and the header. Selecting an appropriate rubber with optimum properties usually involves tradeoffs. Important properties to consider are hardness, retained compression set and tear strength. Compression set is a measure of the ability of the material to spring back after being compressed. This quality is sought in such a seal so that when the joint expands due to temperature change the seal will remain tight. Tear strength has also proven to be important in certain joint designs where localized pressures in the assembly may cause the gasket to split or tear. Hardness affects the difficulty of deforming the material to fill the space in the joint and make good sealing contact. This difficulty directly determines the force required for assembly so that, for a given joint configuration, a high hardness can mandate a high force which creates stresses that are potentially damaging to the tank, the header and the gasket itself. An example of a hard rubber is EPDM rubber in its natural state which has a durometer of 60 or more. To soften the rubber for easier deformability oils are mixed with the rubber to reduce its durometer to 50. Then the tear strength and compression set are likewise reduced. In addition, the gaskets formed of soft elastomers tend to be limp and are difficult to manage during assembly operations whereas the harder elastomer products are stiffer and easier to handle. These tradeoffs have influenced the joint designs of prior tank to header assemblies.
Usually the joint designs have some sort of channel formed in the header plate to receive edge portions of the tank and the elastomeric seal is clamped between them. FIG. 1 is typical of a joint which uses an O-ring seal. A header 10 has a pocket or U-shaped channel 12 formed near its periphery and the tank 14 has an enlarged rim or foot 16 which fits within the channel 12. The foot 16 is offset toward the outside of the tank and has an outer shoulder 18. The channel has an inner wall 20 and an outer flange 22 joined by a bottom 24. A gasket 26 in the form of an O-ring is compressed between the rim 16 and the bottom 24. The edge of the flange is crimped over the shoulder 18 to clamp the assembly together. The O-ring 26 is shown in its undeformed or relaxed state in dashed lines 26'. Gaskets of other cross sectional shapes are known for this purpose, e.g., a double bead, a rectangle, or an ellipse. An example of this style of joint is shown in the U.S. Pat. No. 4,041,594 to Chartet which uses a rectangular cross section gasket and No. 4,316,503 to Kurachi et al which discloses an O-ring. The scheme of using a readily deformable seal such as an O-ring has the advantage that even when made with a hard rubber only moderate forces are needed during assembly to compress the gasket into a good sealing engagement and into conformity to the provided channel space prior to crimping the flange over the shoulder 18. However the space between the foot 16 and the inner wall 20 defines a crevice 28 which will accelerate corrosion attack of the header through creation of an oxygen depletion cell. Aluminum headers are particularly susceptible to this attack. Moreover, a manufacturing problem is that the low cross sectional properties of an O-ring gasket result in a part that is difficult to handle and properly place into the header prior to assembly. Twisting of the O-ring is a common problem.
A common approach to the header-tank seal design is typified by the two seal point configuration shown in FIG. 2. The tank 14 differs from the tank of FIG. 1 in that the heel 30 or inner curve formed at the offset of the foot or rim 16 is positioned close to the wall 20 of the header 10 to define a narrow gap between the heel 30 and the wall 20. Another difference is that the foot has a bead 32 projecting toward the bottom 24 of the channel. The gasket 34 has a rectangular section 36 lying in the bottom of the channel 12 and a tail portion 38 extending through the space between the foot 16 and the wall 20. The relaxed form 34' of the seal is shown in dashed lines. The gasket forms two seals, one between the heel 30 and the wall 20 to prevent the corrosion pocket found in the FIG. 1 design and the other where the bead compresses the rectangular portion 36. The gasket material comprises low durometer elastomers to minimize the force requirements in compressing the gasket during assembly. Also the bead 32 assures that sealing occurs at low gasket compression levels and controls gasket position and material flow during assembly. At high gasket compression levels this bead increases the tendency for the gasket to split under the bead. The gasket extrusion during assembly raises the force required to compress the gasket and also increases the stress level of the gasket material which will reduce the retained physical properties (compression set and recovery) of the gasket. The U.S. Pat. No. 4,289,507 to Cadars et al is an example of this style of seal arrangement.